1. Field of the Invention
The present invention generally relates to an operational amplifier operating at a low voltage.
2. Description of the Related Art
Recently, there has been a demand for a higher SNR (signal-to-noise ratio), lower distortion characteristics, and operability with lower power consumption as the features of the operational amplifiers (driver amplifiers) driving a low load such as a headphone, a speaker and the like. An object of this demand is to extend the voice (music) playback time as much as possible with a small battery.
As an example of an operational amplifier having a higher SNR and lower distortion characteristics, there is a folded-cascode-type class-AB operational amplifier 100 as illustrated in FIG. 4. As illustrated in FIG. 4, the folded-cascode-type class-AB operational amplifier 100 includes:
(1) a current source section having a positive supply voltage PSV0, a negative supply voltage NSV0, and p-channel MOS transistors M101 and M119;
(2) a differential stage having p-channel MOS transistors M120 and M121;
(3) a first cascode connection stage having p-channel MOS transistors M107, M108, M109, and M110;
(4) a second cascode connection stage having re-channel MOS transistors M113, M114, M115, and M116;
(5) an output stage having an n-channel MOS transistor M111, a p-channel MOS transistor M112, a p-channel MOS transistor M117, and an n-channel MOS transistor M118, and capacitor 102 and 103. Input signals input to the non-inverting input terminal INP and the inverting input terminal INN of this folded-cascode-type class-AB operational amplifier 100 are amplified by the first cascode connection stage and the second cascode connection stage, and output as an output signal OUT from the output stage.
In the circuit of FIG. 4, a bias voltage Bias 103 is applied to the gates of the p-channel MOS transistors M107 and M108. A bias voltage Bias 104 is applied to the gates of the p-channel MOS transistors M109 and M110. A bias voltage Bias 107 is applied to the gates of the n-channel MOS transistors M113 and M114. A bias voltage Bias 105 is applied to the gate of the n-channel MOS transistor M111. A bias voltage Bias 106 is applied to the gate of the p-channel MOS transistor M112. Further, the gates of the n-channel MOS transistors M115 and M116 are connected to each other, and further connected to the drain of the n-channel MOS transistors M113.
Generally, the folded-cascode-type class-AB operational amplifiers as illustrated in FIG. 4 have a higher SNR and lower distortion characteristics. However, in the folded-cascode-type class-AB operational amplifiers, there are four or more CMOS transistors connected between the positive supply voltage and the negative supply voltage. Because of this feature, it may become difficult to stably operate at a low saturation voltage. Due to this difficulty, the folded-cascode-type class-AB operational amplifiers may not operate stably.
On the other hand, as an example of an operational amplifier operating at a lower voltage, there is a multi-stage amplification type class-AB operational amplifier 1′ as illustrated in FIG. 5. As illustrated in FIG. 5, the multi-stage amplification type class-AB operational amplifier 1′ includes an amplification stage 2 and a class-AB output stage 3. The amplification stage 2 includes multi-stage (N-stage) amplification sections 7 and 9. The class-AB output stage 3 includes a bias section 4 and an output section 5. The output voltage from the output section 5 is applied to the inverting input terminal (−) of the amplification section 7 (the first-stage amplification section) via a resistor R2.
The multi-stage amplification type class-AB operational amplifier 1′ illustrated in FIG. 5 has the multi-stage amplification sections 7 and 9. Because of this feature, when compared with the folded-cascode-type class-AB operational amplifier 100, the multi-stage amplification type class-AB operational amplifier 1′ may stably operate at a lower voltage. On the other hand, to further reduce the power consumption of the multi-stage amplification type class-AB operational amplifier 1′, the class-AB output stage 3 is required to operate at a minimum voltage.
It is known that the current most consumed in the driver amplifier (operational amplifier) to drive a low load such as a headphone, a speaker or the like is the current in the output section 5 of the class-AB output stage 3. Because of this feature, the power consumption may be greatly reduced when it is possible to operate the output section 5 at the minimum voltages of the absolute values of the positive supply voltage and the negative supply voltage. The output section 5 includes a p-channel MOS transistor M18 and an n-channel MOS transistor M19. Therefore, when the minimum voltages of the positive supply voltage and the negative supply voltage in accordance with the output signal level are given, a stable operation may be achieved. However, even though there is no cascode connection in the amplification stage 2, there are two or more CMOS transistors connected between the positive supply voltage and the negative supply voltage. Because of this feature, it may still be difficult to operate at the minimum voltages of the positive supply voltage and the negative supply voltage.
Further, in the multi-stage amplification type class-AB operational amplifier 1′ of FIG. 5, due to the current consumed in the output section 5 and the impedance of the positive supply voltage and the negative supply voltage, voltage drops of the positive supply voltage and the negative supply voltage occur in accordance with the output signal level. When the voltage drops of the positive supply voltage and the negative supply voltage occur in accordance with the output signal level, it may become difficult for the amplification stage 2 to operate stably. As a result, it may become difficult for the multi-stage amplification type class-AB operational amplifier 1′ to achieve the higher SNR or the lower distortion characteristics.
Japanese Patent Application Publication No. 2007-267016 discloses a cascode-type class-AB operational amplifier providing a higher stable gain even when the supply voltage is lowered.
Further, Japanese Patent Application Publication No. 2001-274642 discloses a low-voltage rail-to-rail CMOS input stage.